Looking at a friends 02 c240 runs rough at idle most of the time but no CE LIGHT?

Has the car been serviced at all?

Spark plugs every 5 years OR 100K miles, should be ready for second spark plug change!

Keeping the plugs in too long causes the wires & coils to fail.

Are the motor mounts causing the engine to vibrate??
That situation can cause the engine to miss as the crank sensor can't keep synchronization with the cam sensor.

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